Electric wave amplification



March 9, 1937. c, w, EE 2,073,477

ELECTRIC WAVE AMPLIFICAT ION Filed Jan. 26, 1925 a mam/0n Patented Mar. 9, 1937' PATENT OF-FlCE ELECTRIC WAVE AMPLIFICATION Charles W. Green,'Millburn, N. 1., auignor, by mesne assignments, to Western Electric Company, Incorporated, a corporation of New York Application January 26, 1925, Serial No. 4,659

31 Claims.

This invention relates to methods and means for amplifying electrical waves.

An object of the invention is to amplify electrical waves without distortion.

A related object of the invention is to reduce even order modulation in balanced amplifiers.

Another object of the invention is to facilitate the operation and maintenance of repeaters in carrier current signaling systems.

The invention provides an amplifier circuit which is particularly adapted for use in carrier current telephone and telegraph systems where several conversations or messages in the form of modulated carrier waves are transmitted over is a single transmission line.

In systems of this type, it is common practice to amplify all the .waves that are transmitted over the line in the same direction in a single repeater, a two-way repeater being provided by duplicating the circuit to provide a path for operation in the opposite direction.

A repeater adapted for this purpose must have in effect a substantially linear characteristic throughout a considerable operating range, in

order to transmit a plurality of frequencies separately and individually without distortion. Balanced amplifiers of the type disclosed in U. S. patent to B. W. Kendall No. 1,544,910, issued July '7, 1925, are commonly employed for this 'purpose. I I

Such a balanced amplifier comprises a pair of vacuum tubes connected in opposition or pushpull relation. The input or control electrodes are so connected to an incoming line that the waves to be repeated are impressed in opposite phase on the respective control electrodes, and

the output circuits of the tubes are so connected through a repeating coil to an outgoing line that an increase of current in one output circuit has a similar effect upon the outgoing line to a simultaneous decreaseof current in the other output circuit.

In theory, the even order products of modulation produced in the usual type of push-pull amplifier, which flow in opposite directions in the two windings of the output repeating coil, are

equal and therefore cancel each other. In practice, however,.it is very difficult to entirely eliminate the even order products. This is be'au'se 50 in practice it is very diflicult to secure two vacuum tubes possessing identical electrical characteristics, and also because imperfect operation results from unbalance due to dissimilarity of the corresponding branches of the divided circuit as- 5 sociated with the vacuum tubes.

Modulation is a term employed to describe the -production of voltages and currents caused by the application of an eiectro-motive-force to a circuit or portion of a circuit or a device possess-- ing the property that the resulting current is not everywhere linearly proportional to the applied voltage producing it. Modulation describes only those component portions of the total voltage and current that are a result directly or indirectly. of the non-linear relation between the applied voltage and the resultin current.

Currents produced in this manner are commonly designated modulation products. For a specific applied voltage the total current is correctly represented as some function of this voltage. I'hat total portion of the current that is a function of an even integral power of the applied voltage is representable by an even order expression. Each such expression-may be separated further into several terms each of which expresses separately and completely the current amplitude of a definite specific frequency. The

component represented by each of these latter terms is an even order modulation product.

Unwanted even order modulation products usually described as crosstalk constitute an operating difliculty that is oftena limiting factor in the engineering application of amplifying repeaters in carrier current 'signaling systems. Such modulation therefore reduces the permissible load carrying capacity of .an amplifier and accordingly makes it necessary to construct and operate and maintain more repeater-stations than would otherwise be necessary. Furthermore, troublesome distortion or crosstalk introduced bythe impossibility of selecting tubes having identical characteristics and further by the impracticability of exactly" balancing the sides of the divided amplifier circuit may reduce the available number of signaling channels.

The present invention provides means for reducing the even order modulation in balanced amplifiers to a minimum. In the preferred form of the invention, a feed-back coupling is employed to return to the common grid circuit branch of a balanced amplifier a portion of the even -crder"'mcdulation products which are in phase in the common branch of the plate circuits. Assuming that the feed-back is properly poled, the resulting even order plate current components will be in phase opposition to the original even order modulation products in the output circuit and these currents therefore cancel each other. a

In the drawing Figs. 1, 2, and 3 are circuit diagrams illustrating three different modifications of the invention, like reference characters indieating corresponding parts of the different figures.

In-Fig. 1, an incoming transmission line comprising conductors I and 2 terminates in the primary winding 3 of an input repeating coil 4, and an outgoing transmission line comprising conductors 3 and 6 terminates in the secondary winding 1 of an output repeating coil 8.

The secondary winding of the input repeating coil 4 is divided into two coils 9 and I0, and the primary winding of the repeating coil 8 isdivided similarly into two coils H and I2. v

The balanced amplifier circuit includes a pair of thermionic three-electrode electron discharge tubes I3 and I4 of any well-known type. Parallel input circuits having a common path l5 and individual paths l5 and I! are connected between the filaments and grids of the two tubes. Similarly, paralleloutput circuits comprising a common path leiand individual paths is and 20 are connected between the filaments and plates of the tubes.

The usual sources of electrical energy 2|, 22 and 23 are provided to properly polarize the grid electrodes, to heat the cathodes, and furnish anode-cathode current, respectively.

The operation of the push-pull or balanced amplifier is well understood, and will not be described in detail. When the incoming waves drive one grid in the positive direction, the other grid is driven correspondingly in the negative direction, each with respect to its associated cathode. An increase of current through the output coil II is accompanied by a simultaneous decrease through the coil 12, and vice versa. The effect of the increase and decrease upon the secondary winding of the output coil 3 will be additive.

The even order products of modulation which are produced in the balanced amplifier fiow in opposite directions in the windings II and I2 and cancel each other out if these components have equal but opposite effects on the secondary 1. However, due to the practical impossibility of securing two tubes which are identical in all respects, and to the difilculty in obtaining exact similarity in the two branches of the divided circuit associated with the tubes, it is very dimcult to completely eliminate these even order products.

The even order modulation products are in phase in the common path it of the plate cir-- cuits and, in accordance with the invention, these components are eliminated by means of a feedback or neutralizing circuit 24, extending from the common path l8 of the output circuits to the common path l5 of the input circuits. The feedback circuit 24 is coupled to the common path l8 of the output circuits by means of a transformer 25 and to the common path l5 of the input cir cuits by means of a variable potentiometer 26.

The feed-back circuit 24 serves to return to the grid circuits of the amplifier a variable portion of the even order modulation products. By thus applying the feed-back across the common lead of the grid circuits the resulting currents in the plate circuits are in phase and, assuming the feed-back to be properly poled, the resulting plate currents will be in phase opposition to the original even order modulation products.

The system described above has been operated efiiciently in amplifying a comparatively small range of frequencies and is very useful, for excause of the variation in ample, in single channel carrier current telephone systems carrying a narrow range of superimposed voice frequency telegraph channels.

When a large range of frequencies are impressed upon the circuit, however, it may be found that the neutralization of the even order modulation products at certain frequencies is incomplete bephase shift with frequency in the transfonner 25. This condition, of course, is inherent in transformers at present available for use in the feed-back coupling, the efliciency of this coupling being somewhat limited by the transformer design.

In order to eliminate the even orderzmodulation products when operating with a large range of frequences the modified system ofIFig. 2 may be employed. In this system an auxiliary tube is employed in the feed-back circuit which is conductively connected between the common leads of the plate and grid circuits.

In Fig. 2 the feed-back circuit includes an electron discharge tube 21, the input circuit of which is connected across a resistance 28 in the common path I8 of the balanced amplifier. The feed-back circuit also includes a pole-changing switch 29 and the grid, filament and plate batteries 30, 3|, and 32, respectively, of the tube 21. By adjusting the variable potentiometer 26 a predetermined portion of the even order products of modulation which are in phase in the common output path I8 are returned to the common input path it, and the plate currents resulting therefrom will be in phase opposition to the original even order modulation products and these currents therefore cancel each other.

In the modified circuit of Fig. 3 the even order products of modulation are balanced out by connecting the common lead of the plate circuits across the output coupling of the amplifier. The neutralizing circuit in this case is connected across the resistance 23 in the common path l8 and across the conductors 5 and 6 in the outgoing transmission line. In this case the balancing circuit may be properly poled by means of a pole changing switch 29, and the current applied to the output circuit of the amplifier may be controlled by means of an adjustable artificial line 33 In the systems described above a slight circuit unbalance produces the same effect in the amplifier output either in the case of even order modulation products formed directly in the tube circuits or even order modulation produced by means of the neutralizing circuit. The even order modulation products are therefore balanced out very effectively.

The invention set forth herein is, of course, susceptible of various other modifications and adaptations not specifically referred to, but included within the scope of the appended claims.

What is claimed is:

1. The method of reducing distortion in an electric wave amplifier having divided input and output circuitswhich comprises impressing a portion of the unbalance currents in the output circuit of said amplifier upon the input circuit thereof in such phase relation that the resulting currents in said output circuit will cancel the uncel the evenorder modulation products in said output circuit.

3. The method of reducing distortion in an electric wave amplifier having divided input and out- "put circuits which comprises impressing a portion of the even order modulation products in the output circuit of said amplifier upon the input circuit'thereof in phase opposition to the original even order modulation products. a

4. The method of reducing distortion in an electric wave amplifier having divided input and output circuits, which comprises selecting the even order modulation components from the output of said amplifier, and applying them to another point in the circuit in such phase and amplitude as to neutralize in the outgoing line the even order components produced in the amplifier.

5. In an electric wave amplifier circuit coupling incoming and outgoing lines, and having divided input and divided output circuits each including a common branch, the method of reducing distortion which comp ises deriving the even order modulation components from the common output branch of said amplifier, and applying them to another point in the circuit in such phase and amplitude asto neutralize in the outgoing line the even order components produced in the amplifier.

6. The method of reducing distortion in an electric wave amplifier having divided inputand divided output circuits each including a common branch, which comprises impressing a predetermined portion' of the currents in the comemon branch of saidinput circuit.

'7. An electric wave amplifier comprising divided input and divided output circuits, and the even order common branches of said circuits to balance out the even order modulation products. 5 10. An electric wave amplifier comprising divided input and divided output circuits each having a common branch, a pair of electron discharge devices connected in push-pull relation in said circuits, and an adjustable feed-back connection 60 including an electron discharge tube connecting common branch of said input circuit to return to said input circuit a portion of the even order products of modulation.

11. An electric wave amplifying circuit comprising two space discharge amplifiers having divided input and output circuits, each of said circuits having a common branch, and means for feeding back the even order modulation products in the common branch of said output circuit to the common branch of said input circuit in such phase relation as to cause the even order modulation products in said output cir- 75 cuit to disappear.

- circuit for feeding back the input .circuit producing a mon branch of said output circuit upon the com-- the common branch of said output circuit to the Y 12. In a signal amplifier having an input and an output circuit, said amplifier having a nonlinear relatlon between output and input waves whereby the amplifier tends'to produce distortion in the form of signal modulation components along with the amplified signal, means for reducing the distortion so output voltage from the output circuit to resultant voltage in theinput circuit opposing the tendency of said amplifier to produce said distortion.

13. In a signaling system, a line for transmit-.

ting waves of a broad line divided into- ,sectlo rid of frequencies, said an amplifying repeater-having an input circuit coupled to an incoming linesection and an output circuit coupled to an outgoing line section, said repeater having a non-linear relation between output currents and input voltages whereby the repeater tends to produce components of new frequencies by interaction between diflerent input frequency components within said broad band, and a circuit feeding back some of the output voltage, including components of said new frequencies, from the output circuit to the input circuit in opposing relation whereby the amplitude of the new frequencies appearing in theoutgoing line section is reduced. I

14. In a balanced amplifier having an input circuit carrying signals to be amplified, and an output circuit containing an outgoing coupling for connection to a load and a branch conjugate to said coupling, said amplifier having two amplifying paths symmetricalwith respect to said branch whereby distortion components flow in said branch tothe substantial exclusion of the signal waves, means reducing the production of distortion components in said. amplifier comprisinga circuit for feeding back waves from said branch to the input circuit of said amplifierin a sense to reduce the amplitude of distortion components flowing in said branch.

15. A signal wave repeater comprising aninput and an output circuit, means to impress signal waves on said input circuit to be repeated, an outgoing circuit for the repeated waves connected between two points in said output circuit between which a difference of potential of the signaling frequency exists, said output circuit having a branch across which a difference of potential corresponding substantially only to modulation frequencies between the impressed signals is developed, and means for reducing signal distortion in saidrepeater comprising a circuit connection between said branch and the input circuit for impressing said modulation frequency components on said input circuit in such phase as to reduce production of said modulation components in. said output.

16.1 In an amplifier circuit, a space discharge device having an input and an output, a source said device, a load circuit connected to said' outproduced comprising a put in series with an external impedance, a

branch circuit connected to said device, said load circuit and said external impedance, across which a difference of potential is developed, representing modulation products of said applied waves to be amplified, and means for reducing the amount of modulation produced in said amplifier circuit comprising a circuit for applying to the input of said device a portion of the voltage developed ratus that reverses the phase of o the extent to which the repeater in said branch, in a phase to reduce production of modulation in said amplifier circuit.

17. In a repeater circuit for simultaneously repeating a, plurality of modulated carrier wa of different frequencies, a pair of space discharge.

devices connected in push-pull relation having input circuits partly common and output circuits partly common, and means to reduce crosstalk between said diflerent waves in said repeater comprising a circuit for impressing on the common input circuit portion voltages derived from the common output circuit portion in such phase as to oppose current variations in said common output portion.

18. In combination, wave translating apparatus, means for supplying to said apparatus waves producing modulation in said apparatus, and means, capable of transmitting waves lying within the frequency spectrum of the supplied waves, for feeding products of the modulation from the output side to the input side of said apparatus in such phase as to reduce thermagnitude of the modulation products appearing at the output side of said apparatus;

19. In combination, wave translating appawaves transmitted therethrough, means; for supp y ng to the input side of said apparatus waves that produce modulation in said apparatus, and means capable of transmitting without phase change waves of all frequencies of a wide frequency range, for

feeding products of the modulation from the output side to the input side of said apparatus in the phase in which they are originally generated.

20. In combination, wave epeating and generating means, an input path for supplying waves to said means, a wave output path for said means, and coupling means, capable of transmitting without phase change waves of all frequencies of a wide frequency range, so coupling said output path with said input path as to prevent the supplied waves from affecting the potential difference between two points on said output path except through the repeating and generating actionsof said means, but to feed waves generated by said means to said input path in such phase as to reduce the intensity of the generated waves in said output path. 4

21. In combination, wave repeating and generating means, an input path for supplying waves to said means, a wave output path for said means,

coupling means, capable of transmitting waves of frequencies lying within the frequency spectrum of the supplied waves, so coupling said output path with said input path as to prevent the supplied waves from aifecting the potential diirerence between two points on said output path except through the repeating and generating actions of said means, but to feed waves generated by said means to said input path in such phase as to reduce the intensity of the generated waves in said output path, a work circuit, and means for supplying to said work circuit a voltage proportional to said potential diiference.

22. In an electric space discharge repeater in which the percentage modulated to unmodulated components produced between wave components simultaneously being repeated is a function of is loaded, the method of increasing the output load capacity for agiven percentage of modulated to unmodulated output which comprises negatively regenerating in said repeater waves including the modulated wave components.

suchphaseas wave components.

24. A repeater for amplifying simul the diiierent frequency components of a signal wave comprising a space discharge tube amplifying system having input and output circuits, and means for reducing distortion in said repeater comprising an output-to-input coupling produc ing in the input circuit resultant waves corresultant waves when repeated into the output circuit reduce said distortion products more than the signal components.

control the action of said repeater as to reduce said components for a given to reduce the magnitude of the modulation components produced in said appara us. 7

.28. Asystemcompridng anelectric space discharge device having an anode, a cathode and a' 30. In combination, wave translating apparatus,meansforapplyingtosaidapparatuswaves aovaam 5 producing modulation in said apparatus, wave balancing and isolating means for deriving, from the modulation components, waves the frequencies of which are exclusive of the frequencies 0! the applied waves, and means for so impressing the derived waves on said'apparatus'as to alter the intensity 0! the derived waves.

31. In a signal wave repeater having input and output circuits, means for reducing distortion comprising means coupling the output and input circuits for the distortion components but not for the signal components.

CHARLES W. GREEN. 

